Population Dynamics ofLaetisaria arvalisand Low-TemperaturePythiumspp. in Untreated and Pasteurized Beet Field Soils

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Abstract
Addition of L. arvalis was highly effective in suppressing reproduction of P. ultimum in pasteurized and untreated field soils planted to table beets (B. vulgaris). The number of germinable propagules of P. ultimum reached a maximum of .apprx. 300 and 6500/g in pasteurized soils amended and nonamended, respectively, with L. arvalis. Addition of L. arvalis to field soil similarly reduced buildup of germinable propagules of low-temperature Pythium spp. In one test, in field soil infested with 455 germinable propagules of low-temperature Pythium ssp. per gram of soil, a maximum of .apprx. 500 and 2000 propagules/g in L. arvalis-amended and nonamended soils, respectively, occurred after 3-wk of incubation. The number of sclerotia of L. arvalis was lower in field soil infested with Pythium spp. than in pasteurized soil, but was still relatively high (30 sclerotia/g of soil) after 16 wk. Significantly greater emergence, lower postemergence damping-off, and subsequently greater survival of table beet seedlings was obtained in soils amended with L. arvalis in comparison to similar treatments lacking L. arvalis. Soils that were planted repeatedly to table beets developed suppressiveness that was characterized by reduced numbers of P. ultimum propagules, increased numbers of L. arvalis sclerotia, and lower seedling disease incidence.

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